1. Field of the Invention
This invention relates to axle drive units and to multi-axle drives.
Particularly the invention provides a tri-axle drive wherein the input torque is split between three driving axles and to a drive unit for use in such a tri-axle drive.
2. Description of Prior Art
It has previously been proposed, for example in patent application GB 2066182A to have a tri-axle drive including various clutch means so that different combinations of axles can be selected for driving purposes. As described in said application the input torque from the engine is split equally between a first axle on the one hand and second and third axles on the other. When all the axles are driven the first axle thus receives 50% of the input torque and the second and third axles receive 25% of the input torque each.
This means in practice that there could be a tendency for the wheels of the first axle to skid. Moreover, for the parts of each axle to be stressed equally the first axle would have to be of substantially twice the capacity of the second and third axles.
It is usual in a tri-axle drive to wish to have all three axles using common parts and therefore in the prior art arrangement, if all the axles do use common parts, either the first axle will be stressed at its load capacity with each of the second and third axles stressed at half its load capacity or if the second and third axles are stressed to load capacity the first axle will be over-stressed.
It has also been proposed in relation to four-wheel-drive vehicles, as distinct from six-wheel-drive vehicles, to have axle drive units which contain an unequal torque split differential gear having two outputs. In these known arrangements the differential gear output receiving the greater torque is connected to the input element of an inter-wheel axle differential gear in the axle drive unit and the output receiving the lesser torque is connected to an output shaft arranged to drive the other axle.
Examples of such arrangements are shown in, for example, UK Patents Nos. 1,093,798; 1,138,942; 1,210,964 and 1,447,875.
In each of these examples the inter-wheel differential gear in the axle drive unit receives the greater amount of torque. The unequal torque split differential gear is a spur type differential gear and the input is to the planet carrier while the output to the inter-wheel differential gear in the axle drive unit is from the annulus gear and the output to the second axle is from the sun gear. These outputs can be concentric with the output from the annulus gear surrounding the output from the sun gear.
Such an arrangement is not suitable for a tri-axle drive unit where one requires to have the greater torque output from the unequal-torque-split differential gear divided between the second and third axles and to have the output delivering the lesser torque connected to the axle whose differential is within the axle drive unit.